The current development towards truly mobile computing and networking has brought on the evolvement of various access technologies that also provide the users with access to the Internet when they are outside their own home network. At present, wireless Internet access is typically based on either wireless LAN (WLAN) technology or mobile networks, or both.
Wireless LAN systems are typically extensions of a wired network, providing mobile users with wireless access to the wired network. In wireless LAN technology, two basic network topologies are available for network configuration: an ad-hoc network and an infrastructure network. An ad-hoc network is formed by two or more independent mobile terminals without the services of a base station, i.e. in an ad-hoc network the terminals communicate on a peer-to-peer basis. An ad-hoc network is normally formed for temporary purposes. The infrastructure network, in turn, comprises one or more wireless base stations, called access points, which form part of the wired infrastructure. In this type of network, all traffic goes through the access points, regardless of whether the traffic is between two terminals or a terminal and the wired network, i.e. the mobile terminals do not communicate on a peer-to-peer basis. The mobile terminals are provided with wireless LAN cards, whereby they can access the wired network, such as the Internet, through said access points, which are mainly located in various hot spots, such as airports, convention centers, railway stations, or shopping malls.
In order to be able to deliver messages, a mobile terminal must first join the network. In this process, which is commonly termed association, the mobile station associates with one of the access points in its neighborhood. At any given instant, a particular access point, i.e. the one with which the terminal is associated, acts as the serving access point for the mobile terminal.
In the following, typical current implementations of the association procedure are discussed briefly. In the simplest embodiment, the terminal selects the first access point it detects. When the terminal is turned on, it starts to scan the channels available in the geographical area in question and selects the first access point it receives. In a more sophisticated approach, the selection is based on the information obtained from the transmissions of a plurality of access points. The terminal utilizes either active or passive scanning in order to detect the access points in the region. In active scanning, the terminal sends a message called a Probe on each channel. When an access point receives a Probe message, it returns a Probe Response to the terminal. In passive scanning, the terminal finds the network simply by listening for the beacon messages, which are periodically broadcast by each access point. Utilizing the above-described active or passive scanning, the terminal scans the channels and examines the information transmitted in the beacon frames or in the Probe Response frames, which contain information about the properties of the access point, such as parameters indicating the security functions of the access point. The terminal determines and stores a parameter termed a RSSI (Received Signal Strength Indicator) which indicates the received signal level on the link to the access point. Having finished the scanning process, the terminal selects the access point with the maximum RSSI, provided that the access point fulfills other requirements set by the terminal. In other words, the terminal assumes that as long as the characteristics of the access point are suitable for the terminal, the access point with the best RSSI provides the best quality of service.
Another important attribute of the WLAN networks is the overlapping of the coverage areas, i.e. cells, of the neighboring access points, since the overlap enables seamless roaming between the cells. When a mobile user with a terminal moves beyond the coverage area of the currently serving access point, the terminal must associate itself with a new access point. This process of transferring an established association from one access point to another is commonly termed re-association.
However, the assumption that the access point with the maximum RSSI provides the best quality of service may lead to a situation where an overwhelming majority of the mobile terminals is associated with a few access points, while some of the access points are substantially idle.
Therefore, load sharing mechanisms have been developed, which result in a more uniform load distribution between the access points, i.e. in a more even distribution of the terminals between all access points. Load sharing mechanisms are based on load information sent by the access points in the beacon or Probe Response frames, the load information indicating the current load of the access point. The load information typically indicates the number of terminals currently associated with the access point. The load information is useful, especially in areas where the cells overlap or in congested areas requiring a multi-cell structure, i.e. where several access points cover essentially the same area.
The above-mentioned use of load information is disclosed in U.S. Pat. No. 6,469,991, for example. This document discloses a wireless communication system in which the beacon message that is broadcast from an access point includes information about the capabilities of the access point, and possibly also load metric information, which generally contains the number of mobile terminals associated with the access point. Based on the information in the beacon message, the wireless terminal chooses the access point with which it wants to associate.
It is further known to transmit various connection attributes from the access points, the selection of the access point being based on the said attributes. International patent application WO01/63842 discloses a method in which the connection is kept in the same network as long as possible. The terminal receives the said attributes from several networks and selects two access points: a first access point, which has the best connection attributes in the network that is the network of the currently serving access point and a second access point, which has the best connection attributes in another network than the network of the currently serving access point. The terminal compares one or more connection attributes of the first and second access points and then re-associates with the second access point if the difference between the connection attributes of the two access points fulfills predetermined criteria. In this way, the connection can be kept in the serving network as long as possible.
However, this method is for roaming use only, as it requires that the terminal already has a serving access point before the method can be initiated.
Generally, a major drawback relating to the above-described known methods for joining the network and re-associating with an access point is that the decision on the correct access point can only be made on the basis of the fixed capabilities and the current load of the access points available for the mobile terminal. Therefore, a number of factors possibly affecting the quality of service within the area of the cell cannot be taken into account when selecting the access point. One such factor is the interference caused by external sources. Possible interference sources in a WLAN environment are Bluetooth devices, for example, which operate on the same frequency band (2.4 GHz) as many WLAN systems, and also other WLAN systems operating independently in the neighborhood. The interference level may also rise if the internal channel separation in the WLAN system is smaller than the optimum 25 MHz.
Furthermore, since the present WLAN networks rely on absolute values of the attributes, such as the load of the access point or the signal level of the serving link, short-term deviations from the overall level of service in the cell may cause undesirable association or re-association decisions. For example, a short-term silent period on a channel or a short-term drop in the load level of the access point may cause such decisions.
The objective of the present invention is to alleviate or eliminate the above-mentioned drawbacks.